A. Field of the Invention
This invention relates generally to electrical transient voltage surge suppression or energy absorbing devices, and more particularly to transient suppression systems that utilize a combination of transient suppression elements including negative resistance breakdown elements such as gas discharge tubes and semiconductor breakdown devices, and non-linear resistance elements, such as varistors or Zener diodes, to provide improved transient suppression.
B. Description of the Prior Art
Various transient suppression circuits are known. Among these are triggered or breakdown devices sometimes known as negative resistance devices that include, for example, gas discharge tubes and semiconductor voltage controlled switches such as a two-terminal device known as a SIDAC. Such breakdown devices have the characteristic that when the voltage applied across their terminals exceeds a predetermined breakdown voltage or trigger voltage, the device switches to a conductive state wherein the resistance decreases and the voltage across the terminals of the device is substantially reduced. The non-linear resistance elements, wherein the resistance is a function of applied voltage and current, include Zener diodes, selenium stacks, voltage-dependent resistors and varistors. Such non-linear resistance elements have a dynamic resistance logrithmically proprotional to the current density within the device. These characteristics vary between devices. For example, the dynamic resistance of the Zener diode varies such that the clamping voltage of the device remains substantially constant over a wide range of current densities. Thus, since the voltage across such non-linear resistance devices tends to rise more slowly than the voltage across a linear resistance, such devices have also been used and are preferred as transient suppressors.
While all of the above-described devices do operate as transient suppressing devices, for the family of negative resistance devices, once a breakdown device is triggered by a transient, its voltage drops substantially, for example, to 15 volts for a gas discharge tube, and on the order of 2 volts for a semiconductor device. If this breakdown voltage is lower than the voltage of the source to which it is connected, the device will continue to draw current, known as follow-on current, after the transient energy has been absorbed. Such follow-on current may be of sufficient amplitude to destroy the device and trip a circuit breaker. The family of non-linear resistance elements, without negative resistance characteristics have the disadvantage that, despite their non-linear voltage characteristics, the voltage across such devices does tend to rise at high current levels.